A method of the above kind is described in U.S. Pat. No. 4,888,877. Here, a rotating swivel unit is described having two motorically-displaceable swivel joints with corresponding angle encoders whose respective rotational axes are perpendicular to each other. The swivel joints of the rotating swivel unit exhibit a reproducible running performance. Corrective values are determined in order to be able to bring the swivel joints of the rotating swivel unit into desired rotational positions without it being necessary to calibrate the sensors anew. The corrective values describe angle position deviations and/or the position of the rotational axes to each other and/or course deviations of the rotational axes. To determine the axis position, it is herein suggested, inter alia, to attach a probe head to the rotating swivel unit which is aligned inclined in space and to move this probe pin about each one of the rotational axes in a plurality of angular positions and to measure the probe ball attached hereto in the particular angular position. The position of the rotational axis results then as a normal to a circular plane through the center point of the circle. The circular plane is adapted in the best possible manner to the measuring points. Furthermore, and as explained above, angular position deviations and/or course deviations of the rotational axes can be corrected. To record the corresponding corrective values, it is suggested to attach plates with four balls to the rotating swivel unit and to measure the center points of the balls for a plurality of rotational positions. From the measured center points of the balls, corresponding corrective values can be determined for each rotational position and these corrective values can then be applied additionally for correction.
What is important in this method is that, to record the corrective values, the spatially-inclined probe ball as well as the plates with the four balls have to be attached to the rotating swivel unit and to record the measurement values, the rotating swivel unit has to be attached to a coordinate measuring apparatus which then records the measured values for determining the corrective parameters.
U.S. Pat. No. 5,778,548 discloses a method of calibrating an optical sensor. The optical sensor is connected via a rotating swivel unit to the measuring arm of the coordinate measuring apparatus via which the sensor can be rotated in two mutually perpendicular rotational axes. The sensor itself includes an illumination unit which projects a laser beam in a line-shaped fanned pattern onto a workpiece to be measured as well as a video camera which evaluates the line-shaped pattern in accordance with the laser triangulation principle.
So that the measured values, which are recorded by the video camera, can be transformed into the machine coordinate system, a rotation matrix is determined, which describes the rotation of the sensor coordinate system in the machine coordinate system, and a vector is determined which describes the offset of the origin of the sensor coordinate system relative to an attachment point of the rotating swivel unit known in the machine coordinate system. Every point measured by the optical sensor can then be transformed into the machine coordinate system via the rotation matrix and the offset vector.
To determine the rotation matrix and the position vector, a cube-shaped test body is provided which had been measured previously into the machine coordinate system via a mechanical probe head. The test body has a bore centrally on the surfaces to be scanned. The Z-axis is defined in the optical sensor arbitrarily as the optical axis of the video camera. The optical sensor is moved approximately along this Z-axis in order to determine the alignment of the Z-axis of the sensor coordinate system. The optical sensor is moved such that it always brings the bore into the center of the image in the different positions. One proceeds in the same manner for the Y-axis of the sensor coordinate system. In this way, the alignment of the Z-axis and the Y-axis and therefore also the X-axis is precisely defined and the parameters of the rotation matrix for rotating the sensor coordinate system in the machine coordinate system and the parameters of the vector, which defines the position of the origin of the sensor coordinate system relative to the point in the machine coordinate system, can be computed.